Single-Site Molybdenum Catalyst for the Synthesis of Fumarate

Article abstract of DOI:10.1002/cctc.201900332

The catalysts with well-defined mononuclear active sites are expected to develop more active catalytic systems for the key chemical transformations. But the rational design of catalyst with stable mononuclear Mo site is still a crucial challenge because of its oligomerization tendency under reaction condition. Herein, Molybdenum catalyst (Mo-8-HQ) with single Mo sites was designed via the pyridine nitrogen and oxygen in hydroxyl of 8-hydroxyquinoline coordinated with Mo atom. The crystal catalyst was stabilized by π-π stacking interaction and hydrogen bonds to form isolated Mo specie. The single-site molybdenum catalyst exhibited excellent catalytic performance in didehydroxylation reactions with high selectively of dibutyl fumarate (86 %) product at mild reaction condition. Deuterium isotopic studies demonstrated that the mechanism feature of didehydroxylation reaction catalyzed by Mo-8-HQ was through concerted cleavage of two C?O bonds process, which could be accelerated by single-site molybdenum catalysts with electron-rich Mo centers.

Full text of DOI:10.1002/cctc.201900332

Accepted Article
Title: Single-Site Molybdenum Catalyst for the Synthesis of Fumarate
Authors: Huifang Jiang, Rui Lu, Xiaoqin Si, Xiaolin Luo, Jie Xu, and
Fang Lu
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Single-Site Molybdenum Catalyst for the Synthesis of Fumarate
Huifang Jiang,[a],[b] Rui Lu,[a] Xiaoqin Si,[a],[b] Xiaolin Luo,[a],[b] Jie Xu,[a] and Fang Lu*[a]
Dedicated to the 70th anniversary of Dalian Institute of Chemical Physics, CAS
Abstract: The catalysts with well-defined mononuclear active sites
are expected to develop more active catalytic systems for the key
chemical transformations. But the rational design of catalyst with
stable mononuclear Mo site is still a crucial challenge because of its
oligomerization tendency under reaction condition. Herein,
Molybdenum catalyst (Mo-8-HQ) with single Mo sites was designed
via the pyridine nitrogen and oxygen in hydroxyl of 8-hydroxyquinoline
coordinated with Mo atom. The crystal catalyst was stabilized by π-π
stacking interaction and hydrogen bonds to form isolated Mo specie.
The single-site molybdenum catalyst exhibited excellent catalytic
performance in didehydroxylation reactions with highly selective of
dibutyl fumarate (86%) product at mild reaction condition. Deuterium
isotopic studies demonstrated that the mechanism feature of
diedhydroxylation reaction catalyzed by Mo-8-HQ was through
concerted cleavage of two C-O bonds process, which could be
accelerated by single-site molybdenum catalysts with electron-rich
Mo centers.
didehydroxylation reactions of polyols.[12] While most Mo species
exhibit moderate performance to form olefin products, and high
temperature in the range of 200-250 °C was normally required.[13]
Polynuclearity in the original Mo structure and aggregation of Mo
catalysts at reaction condition might be the main reasons for the
low activity.[14] Therefore, substantial challenges remain towards
the goal of establishing a new strategy for the synthesis of stable
single-site Mo catalysts.
In this work, 8-quinolinol molybdenum complex (Mo-8-HQ)
catalyst with single Mo sites was synthesized. The pyridine
nitrogen and oxygen in hydroxyl of 8-hydroxyquinoline
coordinated with Mo atom to form isolated Mo specie. The crystal
catalyst was stabilized by π-π stacking interaction and hydrogen
bonds. Tartaric acid (TA) and tartrate with adjacent hydroxyls as
the sugar acid derivates, which can be catalytically converted into
the important building block chemicals, were chosen as the
substrates to study the didehydroxylation reaction mechanism in
view of its simple chain structure feature. Under mild reaction
conditions, the mononuclear Mo-8-HQ catalyst exhibited excellent
catalytic performance in didehydroxylation reactions with a highly
selective dibutyl fumarate (86%) product via the concerted
pathway.
MoO2(acac)2 and 8-hydroxyquinoline (8-HQ) ligand were
used as the starting materials to prepare the 8-quinolinol
molybdenum complex (Mo-8-HQ) by the ligand exchange method.
The experimental detail is explained in Supporting Information.
Through comparing the FTIR spectroscopy (Figure 1a) of Mo-8-
HQ with 8-hydroxyquinoline and MoO2(acac)2, the presence of
two infrared bands at the 894 and 942 cm-1 attributed to
νas(O=Mo=O) and νs(O=Mo=O) in C2V symmetry, respectively,
indicated the cis conformation of the MoO2 (cis-MoO2) moiety.[15]
The infrared broad band centered around 3115 cm-1 in 8-HQ
belong to ν(O-H). This broad band disappeared in Mo-8-HQ
catalyst due to the hydroxyl group in 8-hydroxyquinoline ligand
coordinating with molybdenum atom. This was also confirmed by
the new band appeared at 537 cm-1 in Mo-8-HQ, which could be
assigned to ν(Mo-O). The presence of the band at 497 cm-1 was
assigned to ν(Mo-N) vibration model in Mo-8-HQ. In view of these
characters, we speculated the structure of Mo-8-HQ is 8-HQ
molecules coordinating with Mo atom through pyridine nitrogen
and oxygen in hydroxyl.
Renewable and abundant biomass resources as one of the most
promising feedstocks have received tremendous interests in both
academia and industry.[1] In contrast with fossil resources, the
basic units of cellulose and hemicellulose in biomass contain
polyhydroxy structures resulting in high oxygen to carbon ratio.[2]
Especially, sugar acid, which contains terminal carboxylic
functionalities and polyhydroxy groups, could be the most
promising candidate for synthesizing commonly used dicarboxylic
acid monomers of polymers industry, such as fumaric acid,
succinic acid and adipic acid.[3] The key challenge of this
sustainable route is to develop high-effective catalytic systems
which can selectively remove oxygens in the bioresource. Among
various deoxygenation reactions, for instance high-temperature
pyrolysis[4], hydrodeoxygenation[5], hydrogenolysis reactions[6]
and acid-catalyzed dehydration[7], the didehydroxylation reactions
can efficiently remove two adjacent hydroxyl groups of polyols in
one-step by converting vicinal diols into alkenes.[8]
Molybdenum based catalysts are the best and versatile
catalysts of many industrially important reactions, such as
polymerization reactions[9], olefin epoxidation[10] and olefin
metathesis[11]. Recently, inorganic Mo-based catalysts and a few
of organic Mo complex catalysts have been explored to catalyze
The crystalline nature of the prepared Mo-8-HQ was
characterized by powder X-ray diffraction (XRD). The powder
XRD pattern (Figure 1b) showed that Mo-8-HQ had a new
structure with good crystallinity compared with 8-HQ and
MoO2(acac)2. And its main diffraction peaks could match with
bis(molybdenum bis(8-quinolinol)) oxide (JPCDS No. 26-1950),
which means our Mo-8-HQ catalyst is analogous with
bis(molybdenum bis(8-quinolinol)) oxide, in other words, one cis-
MoO2 moiety probably combined with two 8-HQ molecules to form
one 8-quinolinol molybdenum unit.
[a]
H. Jiang, R. Lu, X. Si, X. Luo, Prof. Dr. J. Xu, Prof. Dr. F. Lu
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian National Laboratory
for Clean Energy, 457 Zhongshan Road, Dalian 116023 (P.R.
China)
E-mail: lufang@dicp.ac.cn
[b]
H. Jiang, X. Si, X. Luo
University of Chinese Academy of Sciences
Beijing 100049 (P.R. China)
Supporting information for this article is available on the WWW
under http://doi.org/10.1002/cssc.201xxxxxx
The valence of molybdenum and the effect of N ligand in Mo-
8-HQ were investigated by X-ray photoelectron spectroscopy
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Figure 1．(a) FTIR spectroscopy of MoO2(acac)2, 8-hydroxyquinoline and Mo-8-HQ. (b) XRD of MoO2(acac)2, 8-hydroxyquinoline and Mo-8-HQ. (c) XPS spectra
of MoO2(acac)2, MoO3 and Mo-8-HQ, proposed construction unit of Mo-8-HQ was inset of (c). (d) SAED pattern of Mo-8-HQ. (e) SEM image of Mo-8-HQ. (f)
Proposed stacking structure of Mo-8-HQ. Molybdenum, oxygen, carbon and nitrogen atoms are black, red, gray and blue, respectively.
(XPS). In the Mo 3d XPS spectra (Figure 1c), two distinct peaks
of MoVI 3d5/2 and MoVI 3d3/2 appeared at binding energies of 232.2
and 235.4 eV, respectively, which were closed to, but lower by ca.
0.5 eV than that of MoVI in MoO3 and MoO2(acac)2 (232.7 and
235.9 eV).[16] The peak shifted of MoVI 3d5/2 and 3d3/2 might be
attributed to the pyridine nitrogen coordination with Mo, which
leads to the electron donating effect from nitrogen to Mo atom and
situates Mo atom in a rich electron condition. Aforementioned, we
deduced from FTIR and XRD, one MoVI in cis-MoO2 moiety
probably coordinated with two 8-HQ ligands via pyridine nitrogen
and oxygen in hydroxyl and assembled into Mo-8-HQ molecule,
i.e. MoO2(oxine)2. Its construction unit was shown as the inset of
Figure 1c.
The selected-area electron diffraction (SAED) pattern of the
bulk Mo-8-HQ particles was shown in Figure 1d. It exhibited
regular diffraction spots, which indicated that Mo-8-HQ posed
single crystal feature. The SEM image showed Mo-8-HQ particles
presented a prism shape and every prism was stacked via several
layers (Figure 1e). The EDS analysis (see the Supporting
Information, Figure S1) revealed that Mo-8-HQ consisted of Mo,
C, N and O, which was also proved by element distribution maps
(Figure S2).
Therefore, the proposed construction structure of Mo-8-HQ
contained one with MoVI as the single active sites,[17] and after
coordinated with two 8-HQ ligands, it will become a more stable
organic Mo complex. According to the reported analogues,[18]
each construction unit of Mo-8-HQ was likely to assemble through
π-π stacking interaction to form layers and through hydrogen
bonds between layers to build up the prism single crystal
structures as shown in Figure 1f.
Various commercially available Mo compounds with different
valence states, such as Mo powder (0), Mo2(CH3COO)4 (II), MoO2
(IV), MoS2 (IV), MoCl5 (V), H2MoO4 (VI), MoO3 (VI), (NH4)6Mo7O24
(VI) and MoO2(acac)2 (VI) were investigated in the
didehydroxylation of tartaric acid. The catalytic activities of these
molybdenum catalysts were examined in glovebox filled with
argon to keep the Mo catalysts stable before reactions. Then the
didehydroxylation reactions were typically performed at 160 °C
over a period of 12 h and biomass-derived 1-butanol was used as
the solvent and reductant.
After the reaction, dibutyl tartrate (DBTA), dibutyl malate
(DBMaA), dibutyl fumarate (DBFA) and acetal were detected
(Scheme 1). DBTA was the esterification product of tartaric acid
and 1-butanol. And 1-butanal as the main oxidized product, which
was generated directly from the hydrogen transfer process of 1-
butanol and trapped as acetal, which has been determined by
GC-MS. The acetal increased following dibutyl fumarate product
as reaction time was prolonged (as shown in Figure S3).
All the results were summarized in Figure 2a, the total yield
of deoxygenate products DBFA and DBMaA varies greatly with
the different valence states of molybdenum. Even though most
Mo compounds were active, it was obvious that molybdenum
catalysts with VI state had a better catalytic efficiency. However,
the normal MoVI, for instance, MoO3 and H2MoO4, were showed
lower product yield. This different behaviour could probably due
to the low solubility of these bulk solid catalysts during the reaction
process. When using MoO2(acac)2 as the catalyst, the combined
yield of deoxygenated species increased to 37%, but it still had
great potential to improve. We designed a type of Mo-8-HQ
catalyst with single molybdenum sites, interestingly, it showed the
highest DBFA yield (ca. 74%, as shown in Figure 2a), a significant
increase compared to other Mo-based catalysts. We further
employed the preparation procedure at air condition, and similar
products yield could be obtained. It means that the harsh reaction
condition without oxygen and water was not the essential element
for our catalyst. When DBTA was used as the starting material for
this didehydroxylation reaction (Scheme S1 and Figure S4), the
reaction proceeded only slightly faster than that of TA, and DBTA
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could be converted into DBFA with the highest yield of 86%. This
result suggested that TA was first inclined to esterification before
undergoing the didehydroxylation process.
Kinetic studies exhibited that tartaric acid was gradually
converted into DBFA product with C=C bond in 160 °C, as shown
in Figure 3a. The yield of dibutyl fumarate and dibutyl malate
increased continuously with MoO2(acac)2 catalyst. Meanwhile,
there was no dibutyl malate as byproduct with Mo-8-HQ catalyst,
and this was similar to methyltrioxorhenium catalyst (MTO)
(Figure S5). It was also confirmed via real-time in situ FTIR, the
didehydroxylation of dibutyl tartrate was monitored real time by in
situ FTIR spectroscopy. The peaks at 1300 cm-1 and 1160 cm-1
corresponded to the C-O stretching vibrations of DBFA[20] (Figure
3b and Figure S6), respectively. It elucidated that DBFA was
produced and accumulated gradually. In addition, except for
DBFA, no other products were observed by real-time in situ FTIR
spectroscopy, consistent with the high selectivity observed by the
GC-MS (the typical total ion chromatogram (TIC) spectrum was
shown in Figure S7).
Scheme 1. Didehydroxylation reaction of tartaric acid using [Mo] catalysts in
1-butanol.
After completion of didehydroxylation reaction, a yellow
powder was separated out from the reaction products by filtration,
then washed with 1-butanol and dried by vacuum. The separated
catalyst was put into a new didehydroxylation reaction of tartaric
acid (TA). The yield of dibutyl fumarate (DBFA) was ca. 52%,
lower than the first run (ca.74%). As shown in Figure S8, the
infrared spectrum of used catalyst could match well with fresh Mo-
8-HQ, except some 1-butanol peaks appears (1-butanol cannot
be removed totally). Thus, it is viable that Mo-8-HQ can be
separated without changing its structure after reaction. So, the
reason of lower DBFA yield with separated catalyst was possibly
that a part of the catalyst dissolved in the solvent after reaction
and cannot be totally separated (the initial catalyst input was ca.
10 mg, and only ca. 0.7 mg was separated). We added a little
fresh Mo-8-HQ into the separated catalyst to make the mass up
to 10 mg, then the yield of dibutyl fumarate (DBFA) was ca. 70%
in a new didehydroxylation reaction.
Figure 2. (a) Didehydroxylation reactions of the tartaric acid with different Mo
catalysts. Reaction conditions: 0.25 mmol L-tartaric acid, 10 mol% Mo catalyst
and 5 mL 1-butanol were added into 38 mL glass vessel, which was operated
in glovebox, then wrapped by Al foil, put into oil bath and kept at 160 °C for 12
h. “*” means preparing procedure was operated in air condition. “**” means
DBTA was used as the starting material. (b) Didehydroxylation reactions of the
tartaric acid with different mole ratios of methyltrioxorhenium (MTO) and Mo-8-
HQ catalysts at 160 °C for 12 h in air.
Rhenium (Re)-based catalyst has been reported as the most
effective catalyst for the diedhydroxylation reaction due to its
special oxophilic ability and multiple oxidation states.[19] However,
the intrinsic properties of Re hampered the implementation of the
didehydroxylation process on boosting industrial scale, as it is a
remarkably scarce element and easy to sublimate in preparing
process. Figure 2b showed the catalytic performance comparison
between Re and Mo-8-HQ catalysts with adding different molar
ratios. Obviously, the amount of catalysts had an effect on the
reaction. Although Mo-8-HQ showed a lower DBFA yield at 1
mol%, it had a similar DBFA yield when the catalyst amount more
than 5 mol%. We thus envisioned that the Mo-8-HQ has the great
potential to replace Re catalyst.
It has been reported that the reduction of the high-valent
metal oxide center and extrusion of the alkene were the rate-
limiting transition state by DFT calculation for the
didehydroxylation of diol.[13b] Aforementioned, the nitrogen as an
electron donor in 8-HQ ligand compelled Mo atom in the rich
electron condition. Thus, it is possible that Mo atom in Mo-8-HQ
will easily lose electrons during its redox process, which is vital for
conquering the rate-limiting transition procedure. In addition, the
literature shows that only strong ligand can form stable Mo
catalyst.[13c] 8-HQ is a ligand with good coordinating capacities,
coordinated with Mo atom through pyridine nitrogen and oxygen
in hydroxyl. Compared with Mo-8-HQ catalyst, MoO2(acac)2 has
very loosely bound ligands, and its coordinating structure will be
destroyed easily during the reaction process, resulting in
deactivation. Moreover, Mo-8-HQ catalyst with single Mo sites
was further stabilized through π-π stacking interaction and
hydrogen bonds in ligand. Therefore, we hold the view that the
excellent catalytic performance of Mo-8-HQ might be attributed to
stable single Mo sites and rich electron condition of Mo centers.
Figure 3. (a) Didehydroxylation reaction of tartaric acid catalyzed by Mo-8-HQ
(10 mol% Mo) and MoO2(acac)2 (10 mol% Mo) at 160 °C, respectively. (b) Real-
time in situ FTIR spectroscopic analysis of the didehydroxylation reaction of
DBTA in 1-butanol. The peaks at 1300 cm-1 corresponded to the C-O stretching
vibrations of DBFA. Reaction conditions: DBTA (2 mmol), Mo-8-HQ (10 mol%),
1-butanol (10 mL), 155 °C.
According to our experiments, except the main product DBFA,
DBMaA was a kind of side product with MoO2(acac)2 as the
catalyst. While using Mo-8-HQ catalyst, the reaction was highly
selective with DBFA as the unique product and no other byproduct
14c, 21]
was observed. As reported before,[13b,
there were two
possible pathways of didehydroxylation for tartaric acid (as shown
in Figure 4a). Path 1 is the concerted pathway via breaking two
C-O bonds, and path 2 is the stepwise pathway via a sequential
“dehydration−reduction−dehydration” procedure. Because of the
reduced steric effect of the primary 1-butanol, the esterification
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step of tartaric acid can be finished quickly.[8c] Herein, we
hypothesize the esterification step cannot influence the
mechanism of didehydroxylation process. Path 1 (shown in Figure
4a with red dot line), adjacent hydroxyls in tartaric acid can be
removed by concerted cleavage of two C-O bonds and generated
DBFA. Path 2 (shown in Figure 4a with blue dot line), at first,
tartaric acid can be esterified and dehydrated to produce
oxaloacetic acid derivate (the tautomeric isomer of 2-
hydroxyfumarate). Secondly, oxaloacetic acid derivate can be
hydrogenated via hydrogen transfer process from 1-butanol to
form DBMaA. Eventually, DBFA can be produced via dehydration
of DBMaA.
of main product DBFA in these experiments indicated that
oxaloacetic acid and malic acid were not intermediate products of
forming DBFA pathway. It was also corroborated that DBFA
cannot be produced from path 2, it should be produced in one-
step by path 1. We used a series of experiments to confirm the
DBFA formation process, and it was very important for
understanding the real mechanism of Mo-catalyzed
didehydroxylation reactions of polyhydroxy compounds.
Therefore, the possible mechanism of the whole reaction
cycle was proposed in Scheme 2. Single-site MoVI catalyst was
firstly reduced to MoIV by 1-butanol. Subsequently, MoIV was
condensed with two hydroxyl groups in tartaric acid through a five-
membered ring to form a Mo-diolate intermediate. Then, the olefin
was extruded from MoIV catalysts via concerted cleavage of two
C-O bonds accompanying with reforming MoVI, which has been
reported as a rate-limiting step. Aforementioned, the single Mo
center was in the rich electron condition after coordinating with
strong 8-HQ ligands, this will facilitate MoIV losing two electrons
and turning back to MoVI. In this regard, it also accelerated the
extrusion of olefin simultaneously, leading to conquer the rate-
limiting transition state.
Figure 4. (a) Two proposed pathways of forming DBFA. (b) Mass spectrums of
DBFA production in deuterium 1-butanol (up) and 1-butanol (down),
respectively.
To determine the formation pathway of DBFA for Mo-8-HQ
catalyst, we designed a series of isotopic experiments. For path 1
with concerted mechanism, the H atoms of C=C in DBFA product
will keep its intrinsic. For path 2 with stepwise process, more than
half of hydrogens on C=C bond in DBFA will come from the
environmental hydrogen donor (as shown in Figure 4a). Above all,
dimethyl fumarate (DMFA) was added into CD3OD, and the
mixture was treated at 160 °C for 12 h. After reaction, the
deuterium content of DMFA was determined by MS. The MS data
showed that the molecular weight of deuterium DMFA was
m/z=150, in contrast to DMFA of m/z=144. This result revealed
the H atoms on C=C of DMFA were stable under reaction
condition (Figure S9). Then, we used deuterium 1-butanol
(CD3(CD2)3OD, 1-butanol with all hydrogen deuterated) as the
solvent and the reductant, tartaric acid as the substrate and Mo-
8-HQ as the catalyst at 160 °C for 12h. The mass spectrums of
the products were shown in Figure 4b. Through analyzing
significant fragment ions peaks in DBFA product after reacted in
deuterium 1-butanol and 1-butanol (m/z=184 and 173, m/z=164
and 155, m/z=120 and 117, m/z=100 and 99), all the H atoms
located at the C=C bond were not replaced by deuterium atoms
from environment. Therefore, we believed that the OH groups
were eliminated through concerted cleavage of two C-O bonds
(path 1) instead of path 2.
Scheme 2. Proposed mechanism of didehydroxylation process catalyzed by
Mo-8-HQ.
In summary, Mo-8-HQ catalyst with single molybdenum sites
was synthesized by ligand exchange method. It exhibits high-
efficiency performance for didehydroxylation of biomass-derived
dibutyl tartrate, and the high yield of dibutyl fumarate product
reached up to 86% at 160 °C. After the reaction, the catalyst could
be separated by filtration. Moreover, the pathway of
didehydroxylation reaction through concerted cleavage of two C-
O bonds was demonstrated via original experimental observation
in our deuterium isotopic studies. It was suggested that the
excellent catalytic performance of Mo-8-HQ for didehydroxylation
reactions might be attributed to the stabilized single Mo sites with
electron-rich Mo centers. The results we presented herein
demonstrated a specific direction to design economic Mo-based
catalysts for didehydroxylation reactions of polyhydroxy
compounds. We believe that our work exploits new opportunities
for using abundant molybdenum catalysts in industrial
applications.
To confirm our inference, intermediate products formed
through path 2, including oxaloacetic acid and malic acid, were
used as the substrates under the reaction condition with Mo-8-HQ
catalyst. Determined by GC, the product was just butyl 2,2-
dibutoxypropanoate from the oxaloacetic acid substrate, which
possibly resulted from the decarboxylation process of oxaloacetic
acid, and the complete conversion of malic acid was almost
produced DBMaA (Figure S10). The similar result was obtained
when MoO2(acac)2 catalyst was used (Figure S11). The absence
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21872139 and 21790331), the
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Keywords: biomass • dicarboxylic acid monomer • single-site
molybdenum • didehydroxylation • cleavage of C-O bonds
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10.1002/cctc.201900332
ChemCatChem
COMMUNICATION
COMMUNICATION
Directing the selectivity in single-site
molybdenum catalyzed synthesis of
Huifang Jiang, Rui Lu, Xiaoqin Si,
Xiaolin Luo, Jie Xu, and Fang Lu*
fumarate
via
enhancing
the
Page No. – Page No.
concerted cleavage of two C-O
bonds pathway
Single-Site Molybdenum Catalyst
for the Synthesis of Fumarate
This article is protected by copyright. All rights reserved.